Within the medical sciences, there is growing interest in the potential utility of purified mammalian antibodies in the diagnosis and treatment of disease. Utilization of the exquisite specificity of particular antibodies for their antigenic determinants has revolutionized the ways in which diseases are described, diagnosed and treated.
For example, tagged antibodies directed against tumor cell surface antigens provide a highly sensitive and specific means for detecting and classifying various cancers. Antibodies also have tremendous potential in the treatment of cancers. One therapeutic application using antibodies involves the administration of purified tumor-specific antibodies that are chemically coupled to cytotoxic agents. A class of cytotoxins that holds particular promise in the treatment of cancers consists of protein toxins from plants. However, progress in the treatment of cancers using chemically coupled antibodies and cytotoxins has been impeded by the lack of a cost effective means for producing these molecules in a pharmaceutically acceptable grade and in commercially acceptable quantities.
Despite recent advances in in vitro antibody synthesis, the production and purification of antibodies remains a costly and time-consuming endeavor. Existing commercial methods require large colonies of laboratory animals or large scale cell culture facilities, each of which is expensive to establish and maintain. In addition, purification of antibodies derived from animals or animal cell culture is a laborious process, because many contaminating biomolecules must be removed from the preparation without destroying the conformational integrity and biological activity of the antibodies.
Obtaining large quantities of purified toxins having potential utility as chemotherapeutics has also proven problematic. The potential for using cloned toxin-encoding DNA sequences to obtain commercial quantities of toxin has not been fully exploited. This is due primarily to difficulties associated with finding a suitable expression system. Generally, toxins expressed in transgenic cells either kill the cells or the toxins are modified in a way that destroys toxicity. Consequently, plant toxin preparations typically have been purified in small quantities from plants in which the toxins are natively produced.
The availability of genes that encode desirable antibodies has caused a shift in focus away from antibody production in animals and animal cells, and toward the goal of obtaining active animal-derived antibodies from plants and plant cell cultures. It is the hope of workers in this field to increase the yield while decreasing the unit cost of purified antibody preparations.
Hiatt and co-workers developed transgenic tobacco plants that contained a DNA construct encoding either immunoglobulin light chain polypeptides or heavy chain polypeptides. Cross-pollination of transgenic tobacco plants yielded F.sub.1 progeny plants that produced functional two-chain antibody molecules. The antibody constituted approximately 1% of total extractable protein. Hiatt and others have also obtained antibody production in a suspension culture of transgenic tobacco cells that contained a single vector encoding both a heavy chain gene and a light chain gene.
PCT patent application WO 91/02066 discloses the transformation of tobacco cells with a recombinant genetic construct encoding a single chain human serum albumin protein molecule fused to an N-terminal plant signal peptide. Human serum albumin was recovered from the medium in which the transgenic cells were cultured in suspension. The heterologous protein constituted approximately 0.02% of the extracellular protein. Expression of antibody-like single-chain variable region fragment (sF.sub.v) proteins that bind antigens has been observed by Owen et al, Bio/Technology, 10:790 (1992) in transgenic tobacco plants at about 0.06-0.1% of total soluble protein.
Expression of novel fusion proteins, or immunofusion proteins, containing an antigen recognition function and an effector function not natively associated with the antigen recognition function has been described. Briefly, using molecular biological techniques, a DNA sequence encoding a desired effector function was introduced downstream from a sequence that encodes an sF.sub.v protein. Expression of these DNA constructs resulted in the synthesis of immunofusion proteins at low levels.
The art has expressed frustration at the inability to achieve high yields of the desired antibodies and protein toxins in a system from which the polypeptides can be readily purified. When a polypeptide is produced at a low level, either in absolute or relative terms, purification of the polypeptide from the plant or plant extract is more difficult.